Heat exchanger for steam generation for a solar thermal power plant

ABSTRACT

A heat exchanger for generating a steam flow for a solar-thermal power plant, including a casing for receiving a casing-side fluid, pipes arranged within the casing for a pipe-side fluid, and a fluid inlet conduit which is connected to an inlet opening for the casing-side fluid and which encloses at least a part of the pipes in such a manner that the fluid inlet conduit is configured as a preheater and/or a flow director for the casing-side fluid entering the casing, wherein heat is transmitted via the pipes from the pipe-side fluid to the casing-side fluid. The casing-side fluid is water, and the pipe-side fluid is a thermal oil or salt.

The invention relates to a heat exchanger for generating a steam flowfor a solar thermal power plant.

Factors such as, for example, an increased economic and politicalawareness for the environment and the increased cost and growingscarcity of fossil fuels have led to a rethinking in the area of powergeneration. New technologies have led to the increased utilization ofregenerative wind and solar energy. In particular solar thermalinstallations with parabolic fluted collectors have meanwhileestablished themselves in large industrial applications so thatinstallations have already been put into operation in the USA andEurope, and further large installations will be added in the nearfuture.

In solar thermal power plants with parabolic fluted collectors, thesunlight is concentrated by means of parabolic reflectors on theabsorber pipes so that the thermal oil found in the absorber pipes isheated to a temperature of approximately 400° C. Thermal energy is drawnfrom the thermal oil with the help of heat exchangers and transferred towater for the purpose of evaporation so that the steam generated therebydrives a turbine for power generation in a connected steam power plantin the conventional fashion. Heat exchangers with U-shaped pipe bundlesin which the separation of the vaporous water from the fluid phaseoccurs in a casing region above the pipe bundle, which is created from aconstructional viewpoint by an expansion of the diameter of the casing,are conventionally used for steam generation.

It has been shown that a separation of the steam in the same casing bymeans of an expansion of the diameter of the casing is disadvantageousin solar thermal power plants and their characteristic cyclic operatingmode. The expanded casing diameter requires an enlargement in the casingwall thicknesses, which has a disadvantageous effect on thethermoelasticity of the heat exchangers, which means that the maximallypermissible temperature gradients during the start-up and thealternating load operation of the power plant are reduced. Accordingly,the availability of the power plant decreases while the risk of materialfatigue increases.

The invention is therefore based on the object of providing a heatexchanger for generating steam for a solar thermal power plant whichreduces or overcomes the aforementioned disadvantages in the state ofthe art.

This object is achieved by the subject matter of independent claim 1.The dependent claims are directed to advantageous embodiments of theinvention.

The heat exchanger in accordance with the invention for generating asteam flow for a solar thermal power plant comprises a casing forreceiving a casing-side fluid and pipes extending inside the casing fora pipe-side fluid. The heat is transmitted via the pipes from thepipe-side fluid to the casing-side fluid, wherein the pipe-side fluid isa thermal oil or salt and the casing-side fluid is water.

The diameter of the casing can be reduced considerably with the help ofthe heat exchanger in accordance with the present invention. The use ofheaders instead of sectional pipe elements reduces the mechanicallyrequired wall thicknesses even further. As a result, the maximallypermissible temperature gradients during the start-up and alternatingload operations can be increased considerably, which leads to a greaterthermoelasticity and availability of the power plant. The increasedthermal elasticity further increases operational reliability, as therisk of material fatigue and thermal cracks is reduced considerably.

The heat exchanger preferably comprises a fluid inlet conduit which isconnected to an entrance opening for the casing-side fluid and enclosesat least a part of the pipes in such a manner that the fluid inletconduit is adapted as a preheater and/or flow director for thecasing-side fluid entering the casing. In accordance with thisembodiment of the invention, the cold water entering the heat exchangercasing first passes through this fluid inlet conduit before it mixeswith the already heated water or water-steam mixture in the heatexchanger. This way, an integrated preheater section is formed, whichproves to be advantageous from a thermodynamic and fluidic point ofview. Moreover, the fluid inlet conduit serves as a flow director.

In a further embodiment of the invention, the fluid inlet conduitencloses approximately ⅛ of the surfaces of the pipes. The fluid inletconduit is preferably constructed in the shape of a box and encloses apart of the heat-emitting pipe surfaces. The fluid inlet conduit canalso be configured in the shape of a cylinder. The ratio of the pipesurface enclosed by the fluid inlet conduit to the entire pipe surfacein the heat exchanger is ⅛. This value can be adjusted in accordancewith the respective application.

The heat exchanger further preferably comprises a fluid outlet conduitwhich is arranged in the region of an outlet opening for the casing-sidefluid in such a manner that the fluid outlet conduit is adapted as aflow director and/or water separator for the casing-side fluid exitingthe casing. This ensures a directed flow of the steam exiting the heatexchanger. Furthermore, the fluid outlet conduit can comprise elementswhich are used for better water or droplet separation.

Preferably, the pipes in the heat exchanger casing are configured as aU-shaped pipe bundle. This way, a large surface area for heattransmission or steam generation and the longest possible dwelling timeof the heat-emitting thermal oil in the heat exchanger are provided in acompact manner. The pipes can also extend in a meandering fashion. Thedimension and arrangement of the pipe bundle can be correspondinglydesigned in an optimal fashion that is adapted to the respectiveapplication.

In a preferred embodiment, the heat exchanger in accordance with theinvention comprises a steam drum which is arranged above the heatexchanger and which is coupled to the heat exchanger by riser pipes anddownpipes. The steam generated in the heat exchanger reaches the steamdrum via riser pipes, from where it is removed for further use orsuperheating. The condensate can be carried off from the steam drum viadownpipes and guided back to the heat exchanger. The arrangement of thesteam drum above the heat exchanger allows a natural circulation.Depending on the application, it is also possible to provide a forcedcirculation by means of a pump.

Preferably, the steam drum comprises a fresh water inlet. This way, aseparate inlet opening for the casing-side fluid (water) on theheat-exchanger side can be dispensed with. The water to be heatedreaches the steam drum in accordance with this embodiment via the freshwater inlet and further via the downpipes to the heat exchanger. Theinvention is explained below in greater detail by reference to theschematic drawings, wherein:

FIG. 1 shows a side view of a first embodiment of the invention;

FIG. 2 shows a front view of the first embodiment of FIG. 1;

FIG. 3 shows a sectional view along the line A-A of FIG. 1;

FIG. 4 shows a side view of a second embodiment of the invention;

FIG. 5 shows a front view of the second embodiment of FIG. 4;

FIG. 6 shows a sectional view along the line B-B of FIG. 4;

FIG. 7 shows a side view of a third embodiment of the invention, and

FIG. 8 shows a front view of the third embodiment of FIG. 7.

FIGS. 1 to 3 show a first embodiment of the heat exchanger 1 inaccordance with the invention. The heat exchanger 1, which is positionedhere horizontally, comprises a casing 10 for receiving a casing-sidefluid (water) and is erected on a support structure 11. Pipes 20 arearranged within the casing 10, the axes of symmetry of which are shownby means of broken lines. This is a pipe bundle with pipes 20 bent in ameandering manner. The hot, heat-emitting fluid, thermal oil, enters theheat exchanger 1 at a temperature of approximately 400° C. and apressure of approximately 20 bar via the oil inlet nozzle 21 and isdirected by means of a distributor 23 into the individual pipes 20 ofthe pipe bundle. After having flowed through the pipes 18, the thermaloil leaves the heat exchanger 1 at a temperature of approximately 300°C. and a pressure of approximately 16 bar via a header 24 and via an oiloutlet nozzle 22 and is re-fed to the absorber pipes of the parabolicfluted collectors (not shown).

The water to be heated enters with a temperature of approximately 300°C. and a pressure of approximately 110 bar through the water inletnozzle 12 or into the heat exchanger 1. The cold water first flows intoa fluid inlet conduit 14 via an inlet opening 13. The fluid inletconduit 14 is designed here in the shape of an angular box and comprisesa rectangular opening 14′ so that the water is necessarily directed uponentrance in the direction of the arrow 15 and only comes into contactwith already heated water or water-steam mixture after passing throughthe opening 14′. The fluid inlet conduit 14 thus serves to direct theflow of the cold water and to preheat the same. The fluid inlet conduit14 encloses a part of the pipes 20 directing the heat-emitting thermaloil so that forced convection occurs within the conduit 14. It hasproven that the ratio of the surface area of the pipes 20 enclosed bythe fluid inlet conduit 14 to the total surface area of the pipes 20 inthe heat exchanger 1 is ideally approximately ⅛.

By means of the transmission of the heat from the thermal oil to thewater, steam is formed in the heat exchanger 1 so that there is amixture of water and steam there, the steam rising in the direction ofthe steam drum 30 on account of the difference in density and the waterbeing found predominantly in the floor region of the heat exchanger 1.The steam makes its way into the riser pipes 31 via the openings 32which are preferably in the vertically upper region of the heatexchanger 1, and further into the steam drum 30. The steam is removedfrom there via the connection 35 and used further. A further heatexchanger (not shown) for superheating the steam is preferablyconnected. The condensate in the steam drum 30 is re-fed to the heatexchanger 1 via the downpipes 33 and the openings 34. The steam drawnfrom the steam drum 30 has on average a temperature of approximately380° C. and a pressure of approximately 108 bar.

FIGS. 4 to 6 show a second embodiment of the invention. The essentialdifference from the first embodiment illustrated above is that the heatexchanger 1 does not comprise a separate water inlet nozzle. Instead,the heat exchanger 1 is supplied with fresh water via the downpipes 33and the openings 34. For this purpose, the steam drum 30 comprises afresh water inlet 36. The production costs can thereby be reducedbecause a separate water connection is no longer required. It is alsopossible to dispense with a fluid inlet conduit 14 because thepreheating of cold water has already taken place in a separatepreheater.

FIGS. 7 and 8 show a third embodiment of the invention. This embodimentis similar to the first embodiment (FIGS. 1 to 3) in principle. Theessential difference is that the pipes 20′ are configured as a U-shapedpipe bundle. As a result, the thermal oil enters the pipes 20′ via thelateral oil inlet nozzles 21 in the direction of arrow 25 via thesectional pipe element 27, gives off heat to the water and leaves theheat exchanger 1 in the direction of arrow 26 via the oil outlet nozzle22. The water to be evaporated enters the heat exchanger casing 10 viathe water inlet nozzle 12 and flows through the fluid inlet conduit 14,wherein the position of the water inlet nozzle 12 and thus also of thefluid inlet conduit 14 is changed in comparison with the firstembodiment. Preferably, the fluid inlet conduit 14 is positioned in theregion of the outlet of the thermal oil.

Temperatures and pressures of the fluid in the heat exchanger can varydepending on the location or size of the power plant.

1. A heat exchanger for generating a steam flow for a solar thermalpower plant, comprising: a casing configured for receiving a casing-sidefluid; pipes arranged within the casing for a pipe-side fluid; and afluid inlet conduit which is connected to an inlet opening for thecasing-side fluid and which encloses at least a part of the pipes insuch a manner that the fluid inlet conduit is configured as a preheaterand/or a flow director for the casing-side fluid entering the casing;wherein a separation of a water vapor from a liquid phase occurs outsideof the casing in a separate steam drum; the casing-side fluid is water;the pipe-side fluid is a thermal oil or a salt; heat is transmitted viathe pipes from the pipe-side fluid to the casing-side fluid; the fluidinlet conduit is arranged entirely inside the casing; the pipes areconfigured as a horizontal meandering pipe bundle; the pipes comprise,on the inlet side, a distributor via which the heat-emitting fluid isdirected into the individual pipes and, on the outlet side, a header bymeans of which the pipes are separated from one another; the steam drumis arranged above the heat exchanger and is coupled by means of riserpipes and downpipes to the heat exchanger in such a manner that anatural circulation occurs; and the heat exchanger is configured for usein a solar-thermal power plant.
 2. (canceled)
 3. The heat exchangeraccording to claim 1, wherein the fluid inlet conduit is configured as aa flow director for the casing-side fluid entering the casing.
 4. Theheat exchanger according to claim 1, wherein the fluid inlet conduitencloses approximately ⅛ of the surfaces of the pipes.
 5. The heatexchanger according to claim 1, further comprising a fluid outletconduit, which is configured as a flow director and/or water separatorfor the casing-side fluid exiting the casing. 6-7. (canceled)
 8. Theheat exchanger according to claim 1, wherein the steam drum has a freshwater inlet.
 9. The heat exchanger according to claim 1, wherein themeandering pipe bundle is configured as a three-way pipe bundle